Radiation shields adapted to flank or surround cryogenic elements or surfaces, especially cryogenic members of machines such as cryopumps, have been provided heretofore in the form of angle strips which lie in a jalousie configuration so as to form an optically obstructing system whereby straight-line passage of radiation to or from the cryogenic surface is blocked by the strips.
Reference will be made to the concept of "optical obstruction" hereinafter and it should be understood that this is intended to mean that the spaced-apart strips are so oriented that they intercept straight-line radiation to or from the cryogenic surface, i.e. are so disposed that a lower edge of one strip lies below or is coplanar with the upper edge of the next underlying strip. To this extent, projections of the strips in a plane parallel to the radiation surface will overlap. It is this type of overlapping which is intended when overlapping is discussed below.
Radiation shields of this type thus include a multiplicity of spaced-apart but superimposed inclined strips of high thermal conductivity held together by a body which itself is cooled, i.e. filled with a cooling medium.
As has been noted, such heat shields are employed to minimize the heat influx to the cold surfaces of cryopumps and the like.
In general the sheet-metal strips are blackened, are disposed in the optical obstruction relationship mentioned previously, but nevertheless form a gas-permeable structure which is cooled by the liquid coolant, for example, liquid nitrogen.
In prior-art systems, these strips have been bent or angled metal strips with a V cross section whose apex angle was usually 120.degree. and the strips were soldered onto the ducts or vessels containing the liquid nitrogen and forming side walls of the radiation shield.
Direct radiation to and from the cryogenic surface was thus blocked effectively by the array of strips flanking the surface while gas permeability, although reduced to about 25% by a properly designed strip array with minimum overlapping, was still quite satisfactory with respect to the savings in liquid helium consumption achieved by this device.
While these devices were found to be highly effective, they were expensive to fabricate and had to be custom-built for each cryogenic installation. Furthermore, once constructed, they could not be effectively altered to change their size or repair or replace the strips. Furthermore, the non-permeability of the sidewalls precludes effective operation of the cryogenic device, e.g. the cryopump, whose intake side frequently was partly obstructed by these side-walls.